Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
The lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like as the anode active material have been considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle life, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life.
Accordingly, methods of using lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a Nasicon structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known Nasicon structure compound, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely studied olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high output voltage of 3.5V and a high volume density of 3.6 g/cm3, as compared to lithium (Li), and has a high theoretical capacity of 170 mAh/g, exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe as an component, thus being highly applicable as the cathode active material for lithium secondary batteries.
However, LiFePO4 disadvantageously causes an increase in internal resistance of batteries due to low electrical conductivity, when used as a cathode active material. For this reason, when battery circuits are closed, polarization potential increases, thus decreasing battery capacity.
In order to solve these problems, Japanese Patent Application Publication No. 2001-110414 suggests incorporation of conductive materials into olivine-type metal phosphates in order to improve conductivity.
However, LiFePO4 is commonly prepared by solid state methods, hydrothermal methods and the like using Li2CO3 or LiOH as a lithium source. Lithium sources and carbon sources added to improve conductivity disadvantageously generate a great amount of Li2CO3. Such Li2CO3 is degraded during charging, or reacts with an electrolyte solution to produce CO2 gas, thus disadvantageously causing production of a great amount of gas during storage or cycles. As a result, disadvantageously, swelling of batteries is generated and high-temperature stability is deteriorated.
In this regard, a method for coating carbon on LiFePO4 is known. However, through repeated experimentation, the inventors of the present invention found that a great amount of carbon should be used in order to obtain the desired electrical conductivity using this method, deterioration in overall physical properties is inevitable during design of batteries and, further, a great amount of carbon used for coating is present in the form of aggregates between particles, thus disadvantageously making it difficult to realize uniform coating.
Accordingly, there is an increasing need for methods to solve these problems.